I would take it one step further. I base this on an understanding--which I will explain--of case law (court opinions).
Instead of asking whether you are free to go, openly declare that you do not consent to the encounter (with police). "Officer, no offense, I know you are just doing your job. But, I do not consent to an encounter with you."
Using a tennis metaphor, you just smacked the ball back across the net. And, he has to play it correctly. Why? Because it forces the cop to have enough information to reasonably hold you while he investigates further (detain you), or release you, or open himself to lawsuit. That is to say, the instant you expressly declare it is a non-consensual encounter, then the cop must have the legal minimum threshold of suspicion to hold you longer while he investigates further. If he doesn't have information to justify that legal minimum threshold (called RAS--reasonable articulable suspicion) and holds you even one second longer, then he is open to lawsuit.
This is a tiny little door the government accidentally left open for citizens. Government spent reams and reams of court opinions determining whether this encounter or that encounter with police was consensual. In doing so, government (courts) waved a giant flag saying, "Hey, if the person just refuses consent, then the only way the cops can force the encounter on the citizen is if the cops have genuine RAS. Or, stated backwards, if the citizen refuses consent, then cops gotta have RAS to force the encounter on the citizen."
The short story is that any cop can contact any citizen consensually, and ask him questions. No minimum level of suspicion required. (Terry v Ohio)
So, if the citizen declares he does not consent to the encounter, the only way the cop can legally force the encounter on the citizen is if the cop has the minimum level of facts to meet RAS.
Thus, I suggest expressly declaring you do not consent to an encounter. It forces the cop to either leave you alone or have enough facts to meet reasonable articulable suspicion. Whether he has those facts or not is not my point. My point is that refusing consent to the encounter is one tiny bit of tactical initiative left open to you by the courts. If the cop does not have those facts and continues the encounter, he is open to lawsuit or formal written complaint.
So, my compantion rule is to declare often (by implication) that it is not a consensual encounter. That is to say, if I say I don't consent to the encounter and the cop doesn't release me instantly, then I just assume I am being detained and start serving across the net:
"Why am I being detained?"
"When will I be released from this detention?"
"The first words out of my mouth were that this is not a consensual encounter."
Repeat in any order.
With any luck the cop will screw up and tell you that you can leave after you answer his questions. (Violation of Title 42, the civil rights act, coercion to waive rights. You've invoked your right to remain silent. But, the cop is going to hold you until you waive the right to remain silent by answering his questions.)
NOTE: Many states and localities* require you to identify yourself to a police officer if he has enough facts to meet reasonable suspicion. Since you can never know for sure whether he has that much facts--he can lie about the facts he has (google "permissibe deception"). Thus, I recommend being very, very careful about refusing to answer an identity question.
*Just because a state law does not exist requiring you to identify yourself to a cop does not mean the local jurisdiction cannot have an ordinance requiring it--and penalizing refusal. I've seen some pretty stiff penalities in local ordinances for refusing to identify yourself to a cop--name and address. Up to a year in jail and a $2500 fine. So, while refusing to answer questions, keep in mind the US Supreme Court has already validated penalties for refusing to identify yourself. See Hiibel vs 6th Judicial District Court.
